The present invention relates generally to controlling optical properties of physical objects using surface treatments, and more specifically to a new class of materials having tailorable thermal emissivity. These materials harness the ability of photonic media to alter the photon density of states in a volume of space.
The thermal emissivity of a body characterizes the flux of thermal xe2x80x9cblackbodyxe2x80x9d radiation emitted by an object. Common materials have thermal emissivity which changes only slightly as a function of the wavelength of the thermal radiation. As a result, the thermal radiation from most objects is roughly proportional to the ideal xe2x80x9cblackbodyxe2x80x9d spectrum, which is the thermal radiation from a body with unit emissivity.
Thermal radiation is of major importance in the passive detection and observation of objects which can not easily be studied using conventional optical methods (i.e., vision via reflected radiation), and especially when active techniques such as radar are inappropriate. Modern equipment to detect such thermal radiation range from simple infrared pyrometers to hand-held thermal scanners designed to detect and image objects whose temperature varies from the (nominally room-temperature) surroundings by less than 1 K, to sensors which can detect the thermal signature of a object re-entering the atmosphere 1000 kilometers away.
Passive thermal radiation detectors can obtain a variety of information about a distant body. At one extreme, the simple presence within the sensor area of a body with thermal signature different from that of the background can be detected as a simple change in the total amount of radiation detected in the waveband to which the sensor is sensitive. Alternately, several sensors, or a tunable sensor, can be used to determine the thermal radiation flux as a function of photon energy. This information can be fit to a theoretical model to obtain an object temperature, or used in some other form as a signature of the object being observed.
At times it is preferable to obtain an image of the object being studied. This is accomplished in many ways, perhaps the best of which currently is a matrix of infrared sensors, each of which studies a single pixel, the sum of the pixels making up an image of the area under study. Again, the image sensor can use differences in absolute magnitude, or can examine the scene at several wavelengths to determine object temperature or thermal signature.
Among the purposes of such passive thermal detection can be the identification of a threat, estimation of size of an unknown body, or obtaining information to guide weaponry to attack said body. In such instances, the observer has a reasonable idea what to look for, and is seeking characteristic thermal signatures. That is, a body having a thermal image or a thermal signature consistent with a known threat will attract the attention of the observer. It is clearly in the interest of a party who wishes to avoid detection, identification, or interception, to find ways of altering the thermal signature of the body.
Traditional methods of camouflage typically have little effect on thermal signatures, which is one of the reasons that such detection techniques are so popular. For example, although coloring a reflecting surface is easily accomplished (e.g., through inclusion of dyes and colorants), introducing significant variations in emissivity as a function of wavelength to alter a thermal signature is much more difficult.
A prior art example of a coating which can provide wavelength dependent thermal emissivity appears in selective solar absorption coatings. Such coatings, usually based on black nickel, which use surface roughness to tailor the thermal radiation properties of the overall system. In brief, a low-emissivity material is given a rough, dendritic surface structure. The characteristic wavelength of thermal radiation from a 6000K source is about 0.5 xcexcm (micrometers), whereas that from a 600K collector is 10 times longer, or 5 xcexcm. If the dendritic surface structure has a characteristic size scale of xcx9c1-2 xcexcm, the roughness will have little effect on the thermal radiation emitted by the collector, which will then exhibit the low emissivity typical of metals. However, the shorter wavelength thermal radiation from the sunlight will penetrate the dendritic surface structure, and therein scatter strongly until essentially all of the sunlight is absorbedxe2x80x94yielding a high absorptivity. Such a structured surface can therefore produce dramatically different thermal radiation properties in different temperature ranges.
Even though some expedients such as were described above allow tailored thermal radiation properties, there is much room for improvement, both in the area of disguising thermal and optical signatures, and in the area of thermal control. The present invention is intended to address these needs.
The present invention relates to methods for fabricating and applying thermal emission control media, wherein said media comprise photonic media. The function of said photonic media is to substantially alter the thermal emissivity of a body. The most convenient method of applying such media is in the form of a paint comprising particles of photonic media, but other modes of application can also be effective.